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Later, in discussing the geology of the Kalahandi State, Walker (1902, p. 10) qualified his earlier formed opinion that the boundary of the charnockite ellipse was an intrusive one by the following
"For some reason as yet unexplained sillimanite-schists do not occur along the western margin in the vicinity of Jeypore'. 'There are, however, evidences indicating a faulted western boundary for the charnockite ellipse, in which case the upthrow of the schists would expose them to denudation, which may account for their absence along the western side of the massif '.
This statement shows that Walker visualized a former covering of sillimanite-schists, or khondalites as he christens them on the following page, over the gneissi plains of Jeypore. This cap of khondalite has according to him now disappeared because it was raised by a fault above the level of the khondalites covering the charnockites further east and has since been totally removed by denudation. The point to be noted is that he considered that the upthrow was on the western side of the fault.
It is not certain what Walker's evidences' of faulting were. He mentions in both of his publications that the most marked direction of strike in the older gneisses is north-west compared with north-east in the charnockite ellipse but that north-easterly slickensiding is also widespread in the older gneisses. He explains the north-westerly strike of the older gneiss as being the imprint of some pre-charnockite earth movement. The slickensiding he considers is evidence that the older gneisses were subjected to the same stresses which gave the charnockite its existing strike. Fermor (1936, p. 143) considers that this slickensiding adumbrates' the presence of a fault at the western margin of the Eastern Ghats. He may be right but I cannot follow his argument myself. My own view is that Walker's evidences of faulting were (1) the sudden change in geology on either side of the charnockite margin and (2) a desire for a fault which would explain the absence of khondalite over the plains of central Jeypore.
Middlemiss has expressed no opinion about the western margin. of the charnockite in his published reports but in an unpublished progress report he makes the following statement:-
'As far as one can deduce from an obscure junction veiled by steep hilly rock-strewn slopes in conjunction with an alluvium
covered plateau, the line of demarcation between the two bands (ie., charnockite and gneiss) is sharp and decisive, though whether it can be spoken of as a fault or fold-fault accompanying the hilly mass of band II to the south-east is problematical '.
He also mentions a little charnockite in the gneissic complex'.
I have recently examined Middlemiss' maps, specimens, slides, and diaries, and though I could not find many of the villages mentioned by him on the topographical maps, I was fairly successful in following his movements. From these it was clear that he was mapping very rapidly and that he made no attempt to follow the boundary between the charnockites and the gneissic complex. He did however make a number of traverses across the boundary at rather wide intervals. His evidence is, therefore, an expression of opinion based on a few traverses at rather wide intervals, an opinion moreover which he did not wish to publish.
The boundary drawn by him is almost a straight line. closely follows the edge of the plain and joins up with Walker's boundary further to the north. There is no indication on the map that he thought the boundary a fault.
The recent mapping of the boundary between Balimela and the Garia river, where Walker's map begins, shows that the edge of the charnockite massif is not a straight line. Results of recent nor does it follow the edge of the plain, as mapping. sketched by Middlemiss. On the contrary it is very irregular (see map). I am satisfied that I had a better chance of achieving accuracy than the older observers, for I had the benefit of excellent new topographical maps, and I was able to check my field results by microscope work on the ground. I was also making a deliberate attempt to plot in the boundary, whereas the older observers were making a reconnaissance of the whole area. The type of boundary shown by me agrees well with that shown by Walker further to the north. Examination of his specimens shews that his map was the result of following the boundary for some distance, and is not based like Middlemiss' map on mere traverses.
As regards the change in strike between the charnockites and the older gneisses noted by Walker, I have not observed any sudden. change. A reference to my map shows that the strike in the older gneisses is somewhat variable. In the majority of cases it seems to be east-north-east like that of the Eastern Ghats. Sometimes
it seems to have been affected by minor charnockite intrusions and runs parallel to their margins. Often it seems to be quite irregular. There is, however, no sudden and marked change along the margin of the main charnockite mass.
Walker's slickensides, which Fermor quotes as evidence of a faulted boundary for the charnockite ellipse, were not noted by me. It is, I think, quite possible that they escaped my observation.
The normal explanation of a boundary like Walker's or mine is that it is an intrusive one. This view is Intrusive nature of the greatly strengthened by the discovery by all three observers of numerous minor charnockite intrusions in the older gneisses lying to the west of the main massif. Contacts between the rocks of the charnockite series and the older gneisses have not been described either by Walker or by Middlemiss. This is largely due to the debris and thick forest growth along the base of the main range. I have recorded one good junction between a large intrusion of hypersthene-granitegneiss and a porphyritic biotite-gneiss in the glen south of Pusapalle (18° 18' 82° 2′).
At the main junction the dividing line between the two rocks is almost vertical, and quite distinct. The gneiss at the contact looks somewhat indurated. The charnockite is abnormally micaceous, but could be matched by micaceous specimens from the interior of the same charnockite mass.
Along the same stream there is a swarm of minor charnockite intrusions in the biotite-gneiss. Here the junction between the two rocks is much less clear, and the one has possibly assimilated the other to some extent.
Microscopic examination of the rocks from either side of the main junction showed that they were both much albitized. This has led to a secondary development in both rocks of biotite, hornblende, albite, quartz, iron-ore, and apatite at the expense of potash felspar and the earlier formed ferro-magnesians. This process, if continued, would have ultimately reduced both rocks to biotitehornblende-granulites almost indistinguishable from one another. As albitization is probably greatest along a junction, it is perhaps to this process, as much as to the prevalence of jungle and debris, that the scarcity of good contacts is to be ascribed.
It would be unfair to use the evidence obtained at one contact as proof that the junction is intrusive, but it is safe to say that
there is nothing in the appearance of this contact to disprove the opinion already put forward that the junction is a normal intrusive
The irregular margin of the main charnockite mass is largely due to the existence of numerous large masses of hyperstheneLarge masses of hy granite-gneiss which jut out irregularly into persthene-granite along the older gneisses lying further to the west. the charnockite margin. These would in my opinion suffice to prove that the contact was intrusive were the charnockite normally of the same composition as its offshoots. This is not so. The main mass of the charnockite is a patchwork of igneous intrusions grading from acid to ultra-basic. All these agree in containing hypersthene and so are related, but some of them have been intruded later than others. In particular the acid intrusions of the western margin of the massif are younger than the main mass.
It is now impossible to say what was the nature of the contact between the main body of the charnockites and the gniesses of the Jeypore-Bastar region before the intrusion of the acid charnockite along this junction, but it is not improbable that it was a thrust-plane or even a boundary fault, though no evidence of such a dislocation can be seen at the present time.
Be this as it may, Fermor makes a mistake in quoting Walker as evidence of a great fault with upthrow to the east. Actually Walker claims that the fault, if any, had an
Fermor's views op
posed to those of all upthrow to the west (see p. 3). Nor does Fermor seem on very safe grounds in claiming that the uplift of the Eastern Ghats as the result of this fault has brought to the surface rocks metamorphosed in the katazone.
As the following excerpts from Middlemiss, Walker and F. H. Smith show, none of the field workers in the charnockite-khondalite region have taken this view.
Middlemiss (1903, p. 24) says The khondalites are regarded as originally sedimentary rocks now completely metamorphosed by the intrusions of charnockite and granite'.
Possible pre-char nockite faulting.
Walker (1902, p. 10) remarks The composition of the sillimanite schist (later called khondalite) taken along with the nature of the rocks with which it is associated compels one to regard this group as para-schists, while their occurring above the charnockites, granitoid gneiss, and other coarse gneisses, all of which appear to
be igneous in origin, suggests that we are dealing with rocks formed by the metamorphism of ancient sediments, very probably by the intrusion of the great igneous masses referred to, and by the action of a mountain building force acting in a line at right angles to the north-north-eastern foliation frequently observed in the rocks of the Kalahandi State'.
Smith (1900, p. 154) takes a 154) takes a somewhat different view. states-The schist evidently represents a more or less metamorphosed series of ancient sedimentary rocks, originally consisting of ferruginous shaly sands and grits with bands of impure limestone. In places the schist passes gradually into these sandy and gritty forms, but usually the latter has been entirely altered chiefly by dynamo-metamorphism, into true crystalline rocks, of which the commonest form is a quartz-garnet schist always rich in sillimanite' (ie., a khondalite).
While I have not seen enough of the khondalite-charnockite junction to be confident of its nature I tend to be in complete agreement with the views of Middlemiss and Walker.
The uplift theory, if correct, would explain the difference between the high grade metamorphism of the khondalites and the rather low grade found in the sedimentaries of Bastar between latitude 18° 22′ and latitude 19°. Recent mapping has shown that further south in Bastar the paragneisses are characterized by the same sillimanite considered to be the hall-mark of high grade metamorphism in the khondalites. Either the supposed faulting is not the cause of the change in metamorphism, or else the fault must die out south of this latitude.
II. THE OLDER GNEISSES IN THE MALAKANAGIRI TAHSIL.
The presence of a great boundary fault along the western margin of the Eastern Ghats was assumed by Sir L. L. Fermor (1935, p. 48) to be the explanation of the sudden change. in the grade of the metamorphism there. Now that it has been shown that there is no real evidence in support of such a fault, and that, if one in fact exists, it is most probably a dislocation which took place before the end of the charnockite period, some other explanation of the supposed sudden change in metamorphism at the margin of the Eastern Ghats is required.